The present invention relates to a new finishing process for quinacridones, quinacridone derivatives, and solid solutions thereof and to their use for the pigmentation of, for example, fibers, plastics, coatings, and printing inks. More particularly, this invention relates to the heat treatment of quinacridone, quinacridone derivatives, and solid solutions thereof, in low concentrations of an organic ester.
Processes for the preparation of quinacridones are well known. For example, S. S. Labana and L. L. Labana, "Quinacridones" in Chemical Reviews, 67, 1-18 (1967); U.S. Pat. Nos. 3,256,285; and 3,317,539). The quinacridones initially formed in such processes, often referred to as crude quinacridones, are generally unsuitable for use as pigments and must be subjected to one or more additional finishing steps that modify particle size, particle shape, and/or crystal structure in such a way that provides good igmentary quality it is well known that pigmentary properties depend not just on chemical structure but also depend on crystal form, crystal modification, and the surface nature of pigment crystals. See, for example, K. Merkle and H. Schafer, "Surface Treatment of Organic Pigments" in Pigment Handbook, Vol. III (New York: John Wiley & Sons, Inc., 1973), page 157; R. B. McKay, "The Development of Organic Pigments with Particular Reference to Physical Form and Consequent Behavior in Use" in Rev. Proq. Coloration, 10, 25-32 (1979); and R. B. KcKay, "Control of the application performance of classical organic pigments" in JOCCA, 89-93 (1989). Some pigments, such as dianthraquinonyl pigments (which, unlike quinacridones, have only one crystal form) can be obtained in pigmentary form with or without finishing steps. E.g., U.S. Pat. Nos. 4,969,954 and 5,078,794. However, such is not the case for pigments such as quinacridones and phthalocyanines.
Although a number of suitable conditioning methods are known to those skilled in the art, the most commonly used methods involve milling the dried crude quinacridone, most commonly in the presence of undesirably large amounts of an inorganic salt that must subsequently be removed. Pigmentary quinacridones can also be obtained by first premilling the dried crude material and then treating the milled material with an organic liquid. Other methods involve a premilling step followed by another milling step using water and small amounts of an organic liquid. Pigmentary quinacridones can also be obtained by heat treatment of crude presscake in large quantities of solvent. Additives have been added during the milling or solvent treatment steps to further improve pigment properties.
When using organic solvents for conditioning a pigment, the water that is present in presscakes and the air that is present in powders is replaced by the solvent, a process known as "wetting out." Because of the differences in physical properties, different types of pigments react differently to the wetting-out process. See, for example, T. B. Reeve and W. L. Dills, "Pigment Dispersions and Rheology in Plastics," and T. A. Langstroth, "Pigment Flushing" in Pigment Handbook, Vol. III (New York: John Wiley & Sons, Inc., 1973), pages 441-446, and 447-455, respectively. It is often necessary to aid the wetting-out process by using surface-active agents, but even here the differences in the pigments affects the selection of surface-active agents. Therefore, many methods have been reported for the finishing of quinacridone pigments with organic solvents.
For example, U.S. Pat. No. 2,857,400 discloses a method in which a crude pigment is milled in the dry state and then agitated in an organic liquid. U.S. Pat. No. 3,017,414 discloses a method in which crude pigment is premilled in a ball mill and then treated with an emulsion of water and certain specified organic liquids in the presence of a small amount of a surfactant. U.S. Pat. No. 4,094,699 discloses the treatment of a premilled crude quinacridone with an aqueous alkaline solution containing surfactants. U.S. Pat. No. 4,298,398 discloses a similar method in which a premilled crude quinacridone is treated in an aqueous alkaline solution containing cedain quaternary ammonium compounds. U.S. Pat. No. 3,287,147 discloses a process in which crude quinacridone is prepared in a finely divided or crystalline form (e.g., by either acid pasting or ball milling) and then converted to a neutral aqueous paste that is heated at 150.degree. C. to 300.degree. C under pressure using special equipment. U.S. Pat. No 4,455,173 discloses a process in which crude quinacridone pigments are acid pasted or ball milled and then milled in an organic liquid, preferably in the presence of a 2-phthalimidomethylquinacddone particle-size growth inhibitor. U.S. Patent No. 5,084,100 discloses a method in which crude quinacridone is ball milled in the presence of aluminum sulfate and esters of alkanedicarboxylic acids. U.S. Pat. No 4,541,872 discloses a method in which premilled crude quinacridone is ball milled in dilute aqueous base, an aqueous basic salt solution, or an aqueous solution of a base and a salt in the presence of a small amount of solvent, optionally in the presence of particle size growth promoters or inhibitors. Although these procedures give desirable results, they are generally time-consuming and relatively expensive.
Crude quinacridones can also be conditioned with organic solvents to give dispersible pigments. For example, U.S. Pat. No. 4,895,948 discloses a one-step finishing process in which a crude quinacridone is ball milled in an alcohol containing a base. U.S. Pat. No. 5,084,573 also discloses a one-step finishing process in which 2,9-dichloroquinacridone is stirred in heated polar organic solvents containing, as essential ingredients, cedain long-chain thiol compounds and a base. U.S. Pat. No. 5,095,056 discloses a process for conditioning 2,9-dichloroquinacridone using large quantities of polar solvents, including esters such as methyl benzoate, at temperatures above 50.degree. C. Although bases and other such additional compounds are not required, the polar solvent is used in an amount that is about 3 to 20 times the weight of the pigment. Water can be tolerated but is not preferred. U.S. Pat. No. 3,256,285 similarly discloses a process for finishing aqueous pastes of quinacridones in large quantities of organic solvent (4 to 10 times the amount of pigment) at temperatures of 80.degree. to 150.degree. C. When using high-boiling or water-immiscible solvents (such as methyl benzoate and methyl salicylate) according to U.S. Pat. No. 3,256,285, the presscake must be washed with low boiling organic solvents, thus requiring the disposal of significant quantities of organic liquids. The present invention, in contrast, provides satisfactory results when aromatic esters are used as the organic solvent in quantities as small as 0.6 times the amount of quinacridone and without the need for special additives.
It was an object of the present invention to obtain high quality quinacridone pigments while avoiding the disadvantages of the known methods. The present invention unexpectedly provides an advantageous method for preparing such quinacridone pigments. First, it is not necessary to dry and grind the crude quinacridones before carrying out the finishing process of the invention, thereby reducing production time and costs. In addition, the organic solvents used in the process are esters that can be hydrolyzed to the constituent carboxylic acid and alcohol components. Because the hydrolysis products are water soluble, the pigment prepared according to the present invention need not be washed with an organic solvent. Because only small amounts of the organic ester solvent are required, the process of the present invention avoids the need to recover or recycle the solvent and reduces the amount of organic materials released into the environment. Finally, high quality pigments can be obtained even without using additives.